Unless otherwise qualified, silicon carbide shall mean stoichiometric silicon carbide.
Carbon-rich silicon carbide shall mean deposits in which the ratio of silicon to carbon shall be in the range of 0+to 1.
High-strength and/or high-modulus filaments or strips shall mean structures having a tensile strength of 300 ksi or greater and a tensile modulus of 40,000 ksi or greater.
The present invention is applicable to silicon carbide and carbon surfaces of all shapes and sizes. It is particularly important for filaments, thin strips, and the like. The following discussion will be directed to filaments as a typical example.
Composite materials in plastic or metal matrices reinforced with high-strength, high-modulus filaments such as boron and silicon carbide are finding increased popularity in structural applications. In particular these types of composites are useful where high-strength and stiffness with accompanying low weight is desired.
The present state-of-the-art silicon carbide filament or thin strip contains a refractory core, generally tungsten or carbon. The core may include an intermediate buffer zone followed by a relatively thick layer of silicon carbide. In general the silicon carbide and buffer zone are made by means of hydrogen reduction and chemical vapor deposition processes wherein gases containing silicon and carbon are decomposed and deposited on the core. The thickness of the silicon carbide coating is directly related to the deposition time and temperature.
An important use of said silicon carbide coatings is in connection with high-strength, high-modulus silicon carbide filaments of the type described in U.S. Pat. No. 4,068,037. In this patent there is described a silicon carbide filament formed on a carbonaceous core. In other applications the silicon carbide coating is deposited on a tungsten core.
The aforementioned referenced patent, U.S. Pat. No. 4,068,037, is the closest art known to Applicants in relation to the present invention. The filament described in the referenced patent represents the state of the art. In particular, it will be noted that the filament described in the patent contains a carbon-rich silicon carbide outer coating which is important for maintaining overall filament strength and stiffness. However, this outer coating makes it very difficult to incorporate these filaments within metal matrices such as aluminum, titanium, as well as epoxy matrices because the matrix material does not bond well to the carbon-rich outer layer.
The outer carbon-rich silicon carbide layer of the state-of-the-art silicon carbide filament discussed above has been physically characterized as a layer where the ratio of silicon to carbon varies from one at the interface of the carbon-rich layer with the stoichiometric silicon carbide layer to zero at the exterior surface of the filament. In other words, the exterior surface of the carbon-rich layer is essentially pure carbon.
The industry has long known that it is extremely difficult to incorporate carbon filaments within plastic and metal matrices. Carbon is, in one instance, highly reactive. Prior attempts to incorporate such state-of-the-art silicon carbide filaments containing carbon surfaces and/or carbon filaments within aluminum or titanium matrices by hot-molding have been less than desired. In most cases the resulting composite is not very strong because the molding process has greatly weakened the filaments. Additionally, carbon is not readily wet by aluminum or titanium, and even the common plastic matrices such as epoxys. As a result, composite properties suffer.
The difference in properties of the filament in composites between the heretofore state-of-the-art practice and the practice enumerated in this application will become readily apparent with reference to the chart that is provided herein.
It is an object of the invention to provide a surface treatment for stoichiometric silicon carbide which enhances the wetting capability of the silicon carbide without deleterious effects on the strength of said silicon carbide.
It is another object of the invention to provide a surface treatment for carbon which also enhances its wetting capability without deleterious effects on its strength.
It is yet another object of the invention to provide a method of making a high-strength, high-modulus silicon carbide filament.
In accordance with the invention, there is provided a surface treatment for stoichiometric silicon carbide and/or carbon comprising a carbon-rich silicon carbide layer overcoating the stoichiometric silicon carbide or carbon. The ratio of silicon to carbon of the carbon-rich layer varies from one at the interface with stoichiometric silicon carbide to near zero at the interior to a value substantially greater than zero at the surface remote from said interface. The carbon rich silicon carbide treatment for carbon surface has a silicon to carbon ratio of zero at the carbon surface to a value greater than zero in the remote surface.
It is hypothesized that silicon carbide is particularly sensitive to the presence of non-stoichiometric silicon carbide or impurities. I. T. Kendall, Journal of Chemical Physics, Vol. 21, pg. 821 (1953). Since both Kendall and K. Arnt & E. Hausmanne in Zeits Anorg Chem., Vol. 215, pg. 66 (1933) have found no evidence of non-stoichiometric silicon carbide, it is hypothesized that the excess carbon appears in the silicon carbide as an impurity. The properties of silicon carbide are particularly sensitive to the presence of impurities such as carbon.
Though the precise strength of carbon-rich silicon carbide may not be known with certainity, regions where quantitatively there is an excess of carbon have been observed.